472 Mr. W. H. Preece [June 4, 



It tends to reduce the wave lengths — Righi's waves are measured in 

 centimetres, while Hertz's were measured in metres. For these 

 reasons the distance at which effects are produced is increased. 

 Mr. Marconi uses generally waves of about 120 centimetres long. Two 

 small spheres, a and b, are fixed close to the large spheres, and con- 

 nected each to one end of the secondary circuit of the " induction 

 coil " C, the primary circuit of which is excited by a battery E, 

 thrown in and out of circuit by the Morse key K. Now, whenever the 

 key K is depressed sparks pass between 1, 2 and 3, and since the 

 system A B contains capacity and electric inertia, oscillations are set 

 up in it of extreme rapidity. The line of propagation is D d, and the 

 frequency of oscillation is probably about 250 millions per second. 



The distance at which effects are produced with such rapid 

 oscillations depends chiefly on the energy in the discharge that passes. 

 A 6-inch spark coil has sufficed through 1, 2, 3, up to four miles, 

 but for greater distances we have used a more powerful coil — one 

 emitting sparks 20 inches long. It may also be pointed out that this 

 distance increases with the diameter of the spheres A and B, and it 

 is nearly doubled by making the spheres solid instead of hollow. 



The Receiver. — Marconi's relay (Fig. 2) consists of a small glass 

 tube four centimetres long, into which two silver pole-pieces are 

 tightly fitted, separated from each other by about half a millimetre 



a thin space which is filled up by a mixture of fine nickel and 



silver filings, mixed with a trace of mercury. The tube is exhausted 

 to a vacuum of 4 mm., and sealed. It forms part of a circuit 

 containing a local cell and a sensitive telegraph relay. In its 

 normal condition the metallic powder is virtually an insulator. 

 The particles lie higgledy-piggledy, anyhow in disorder. They 

 lightly touch each other in an irregular method, but when electric 

 waves fall upon them they are " polarised," order is installed. Tliey 

 are marshalled in serried ranks, they are subject to pressure — in 

 fact, as Prof. Oliver Lodge expresses it, they " cohere " — electrical 

 contact ensues and a current passes. The resistance of such a space 

 falls from infinity to about five ohms. The electric resistance of 

 Marconi's relay — that is, the resistance of the thin disc of loose 

 powder — is practically infinite when it is in its normal or disordered 

 condition. It is, then, in fact, an insulator. This resistance drops 

 sometimes to five ohms, when the absorption of the electric waves by 

 it is intense. It therefore becomes a conductor. It may be, as sug- 

 gested by Prof. Lodge, that we have in the measurement of the variable 

 resistance of this instrument a means of determining the intensity of 

 the energy falling upon it. This variation is being investigated both 

 as regards the magnitude of the energy and the frequency of the 

 incident waves. Now such electrical effects are well known. In 

 1866 Mr. S. A. Varley introduced a lightning protector constructed 

 like the above tube, but made of boxwood and containing powdered 

 carbon. It was fixed as a shunt to the instrument to be protected. 

 It acted well, but it was subject to this coherence, which rendered 



